1. Field of Invention
The invention relates to an apparatus for the longitudinal stretching of a film web and a corresponding method.
The properties of thermoplastic films can be changed intentionally through their stretching. Such properties include, for example, the transparency or the strength. Such stretching can occur in both the lateral and/or longitudinal directions of the film web and can be effected directly in-line after the extrusion process. However, the extruded film may also be initially rolled up and the apparatus for the longitudinal stretching can be used after reprocessing (“offline processing”).
2. Description of the Prior Art
FIG. 1 of the document WO 2006/063641 A1 shows a multi-stage apparatus for the longitudinal stretching of a film web. The features of such an apparatus include:                at least one heater in which the film web can be heated,        at least one cooling device in which the film web can again be cooled down, and        at least one stretching apparatus in which the film web is stretchable in its longitudinal direction,whereby the stretching apparatus has a first transport roller running at a first peripheral speed in the stretching operation and, looking along the transport path of the film web and following the first transport roller, a second transport roller having a second peripheral speed, whereby the second peripheral speed is higher than the first peripheral speed.        
In general, such an apparatus also has annealing or tempering rollers that are positioned between the stretching apparatus and the cooling rollers. These latter rollers often already have a lower temperature than the heating rollers, but are so tempered that they avoid too rapid a cooling of the web. The difference between the cooling rollers and annealing rollers is often achieved by hot oil flowing through the annealing rollers and water flowing through the cooling rollers.
It is a fact that most of the actual stretching process occurs between the actual stretching rollers.
The longitudinal stretching, i.e. a stretching of the film web in its transport direction, takes place as a result of the higher peripheral speed of the second transport roller compared to the peripheral speed of the first transport roller. Simultaneously, the thickness of the film web decreases. However, in the heating device, the film web is first heated. After it has been stretched, the film web passes through the cooling device to again cool the film web to a temperature well below the plasticizing temperature. Film webs used with such an apparatus for longitudinal stretching are preferably processed from thermoplastic materials such as polystyrene, polypropylene or polyethylene.
The actual stretching process takes place between the two transport rollers, whereby this process begins immediately after releasing the film from the first transport roller.
It has been determined from thickness measurements of film webs, however, that thicknesses vary greatly in the lateral direction. This means that, looking in the lateral direction, the film web is stretched to different degrees. Films with varying thicknesses in the lateral direction, however, are often not desired.